Mobile satellite communications systems are being employed worldwide. In general, previous systems have been regional in nature. However, global systems are being proposed and introduced. These global systems offer the potential for international roaming and, as a result, a need will exist for user terminals which can allow worldwide roaming. That is, a need will exist for a user terminal that can be used to make and receive calls without regard for geographic, economic, or political boundaries.
Single mode satellite user terminals (i.e., satellite only) can allow worldwide access. However, with the wide spread deployment of cellular, Personal Communications Systems (PCS) and other terrestrial mobile systems there will be a logical need to inter-work with such systems. For example, in such systems, and for dual mode terminals (e.g., analog/digital), it is often the case the digital mode is preferred, and is automatically selected where available.
Such inter-working is cost effective and may increase user quality of service in urban and suburban environments, while the satellite service can be used in rural areas and areas where terrestrial mobile service is not available or economical.
In order to accomplish such inter-working a dual mode user terminal may be required. That is, a user terminal capable of operating in either the satellite or the terrestrial cellular environment.
It is envisaged that mobile users will be roaming in both of these environments. The terrestrial environment is generally limited to those service areas which have land-based repeaters (towers, etc.) or other local area repeaters (e.g., balloons or aircraft). Necessarily, these mobile service areas are rather small, and are generally located in cities, towns and along heavily traveled roads. The satellite system, on the other hand, provides ubiquitous coverage and includes both the terrestrial mobile service areas and non-terrestrial service areas.
Terrestrial mobile systems do not have perfect coverage, especially at the fringe of cell areas, and furthermore many have "holes" or "gaps" within their service areas due to terrain blocking or other reasons. Users of terrestrial mobile systems can experience dropped calls and service when located within these fringe areas and holes. Satellite systems can fill these holes and fringe areas if the user terminals can inter-work sufficiently to assure proper technical connection and switch-over from one system to another.
At present, there are no currently existing or proposed terrestrial/satellite communications systems known to the inventors that adequately address these important needs.
It is noted that in an article entitled "An Integrated Satellite-Cellular Land Mobile System for Europe" (1989), E. Del Re describes a proposal for integrating a satellite communication system with the European GSM terrestrial cellular system. In the Del Re approach is it desirable to maintain common protocols (layers 2 and 3) between the satellite and GSM systems, with the majority of the differences being in the layer 1 (e.g., RF bands and synchronization procedures).
In U.S. Pat. No. 5,073,900, entitled "Integrated Cellular Communication System", A. Mallinckrodt describes a cellular communications system having fully integrated surface and satellite nodes. That is, space node satellite cells can overlap ground cells. A system network control center is used to designate handling for a particular call to one of a satellite node or a regional node control center. Mallinckrodt mentions that in one embodiment the user can select whether to use the satellite link or the ground-based link.
Neither Del Re or Mallinckrodt is seen to provide a satisfactory solution to the problem of providing full inter-working between a satellite communication system and one or more existing terrestrial communication systems.